GMRS Base Station Antenna selection

[prcguy]

There are engineered answers to your mounting question then there are our opinions here on RR, so take these with some caution. With that out of the way, for my personal use at my house I would have no problem using something like 1 5/8" fence top rail as a mast up against the house, anchored to the eaves with up to 10ft of that unguyed above the roof and a 9ft tall Laird FG4607 on top. I would not do that for a commercial customer.

I currently have a US Tower MA40 telescoping 40ft tower bolted to a concrete pad and my beefed up eaves, which are about 9ft off the ground. That is about 31ft of tower sticking up unguyed with a 17ft antenna on top of that. I think it will easily withstand a 100mph wind.

Apologies: I missed your post recommending the higher gain 4607.
It may make redundant my post below.
How high can I safely go on a pole mount - assuming that I twill use two brackets for the end wall perhaps space 2ft apart - meaning that there could be nearly 3ft of pole below the roof apex on the end wall..

 

[WRFM426]

I have 10' of mast from the top of my repeater tower at my house. I would say 10' its pretty much the max without guy wires.

Yup, That is what I was thinking re height of pole: top of antenna would be nearly 8-9ft above that - so that is about 15ft total height (~7ft of exposed pole + ~8ft of antenna).

 

[WRFM426]

I've had past conversations with at least two different antenna mfr application engineers and they all agree a 10dBd gain antenna is good for up to about 1,000ft AGL. I doubt if your on a hill that approaches that height, so a 7dBd gain antenna should have no problems with weak or dead zones around the base of a hill. The 33deg 1/2 power beamwidth on the 7dBd gain Laird FG4607 means that at 16.5 degrees above and below the horizon the gain will have dropped by half or 3dB. Those would be the 4dBd gain points of the antenna at those extreme up and down angles.

The more gain an omni antenna has the narrower the half power beamwidth will be, but that doesn't mean it cuts off sharply and there is no signal at all above or below those half power points. An antenna has a main lobe, which is what is advertised and there lots of smaller lobes with lower gain that fill in the areas outside the main lobe to some extent. If I were in your type of terrain I would try and find a 10dBd gain stick, but those can be hard to find at a good price and I would be satisfied with a 7dBd gain model which is more affordable. That's why I have a couple of new ones in the garage along with a dozen other types for those pop up repeater projects.

I will be proceeding with your advice taken into account.
I will report back with results once the system is up and running..!

 

[prcguy]

The end of the first paragraph below should read "Those would be the 3dBd gain points of the antenna at those extreme up and down angles.

I've had past conversations with at least two different antenna mfr application engineers and they all agree a 10dBd gain antenna is good for up to about 1,000ft AGL. I doubt if your on a hill that approaches that height, so a 7dBd gain antenna should have no problems with weak or dead zones around the base of a hill. The 33deg 1/2 power beamwidth on the 7dBd gain Laird FG4607 means that at 16.5 degrees above and below the horizon the gain will have dropped by half or 3dB. Those would be the 4dBd gain points of the antenna at those extreme up and down angles.

The more gain an omni antenna has the narrower the half power beamwidth will be, but that doesn't mean it cuts off sharply and there is no signal at all above or below those half power points. An antenna has a main lobe, which is what is advertised and there lots of smaller lobes with lower gain that fill in the areas outside the main lobe to some extent. If I were in your type of terrain I would try and find a 10dBd gain stick, but those can be hard to find at a good price and I would be satisfied with a 7dBd gain model which is more affordable. That's why I have a couple of new ones in the garage along with a dozen other types for those pop up repeater projects.

 

[WRFM426]

So here’s another question: lightning arrestor.
Any preferred brands, and where is the most appropriate place to install it?
Should it, for example screw into the antenna before coax, at the other end of the coax, or midway..?
Associated question re electrical grounding of arrestor and the antenna: is there a Best Practice for how to achieve grounding of both items?

 

[tweiss3]

I use Polyphaser. You should use them right before the coax enters the house.

I have hardline coming down from the antenna, then it makes a drip loop and enters a utility box on the side of the house and connected to the polyphaser where the polyphaser is bolted to the grounding plate inside. Grounding plate has 2AWG ground wire bolted to it, doing directly down to the 8' grounding rod. Then I have another piece of coax with a 90 degree fitting on it attached to the polyphaser going into the house to the equipment inside.

 

[bill4long]

Keep in mind, grounding and Polyphasers are not going to protect you from a direct hit. They offer some protection against indirect strikes (which results in a lots of static electricity near the strike.) But if you get a direct hit, it will be total devastation of your equipment and coax, and probably total devastation of your antenna. We're talking 300 billion coulombs of electrons per average strike. (Think 300 billion watts for one second.)

If you want full protection against a direct hit, unplug all your equipment from the antenna and AC power when lighting storms are around.

Hams have been debating the pros and cons of grounding your system since time immemorial. (I'm on the "anti" side. There's probably an existing thread that covers the topic.)

 

[prcguy]

I think its most effective to place a lightning arrestor at the cable entry point of the building, but you would need your main building ground available at that same spot. If this is a home installation be aware that a lightning arrestor will not protect much from a direct strike unless your entire house and ground system is designed and built specifically for lightning mitigation. Otherwise a direct hit will probably destroy much of the electronics in your house. Its better to know this and disconnect things that to believe you are actually protected.

Do a search on "NEC article 810" and you will get lots of proper advice for grounding an antenna and arrestor to meet code. It will inform you of things like any additional ground rods must be bonded to the main house ground rod, etc. I see Polyphaser arrestors used at major communications sites and I also use them.

 

[WRFM426]

Another Question regarding ordering equipment: has anyone used Digi-Key (digikey.com)? Seem to have good prices and sensible shipping costs..

 

[prcguy]

I bounce between Digikey and Mouser for small parts but not antennas.

Another Question regarding ordering equipment: has anyone used Digi-Key (digikey.com)? Seem to have good prices and sensible shipping costs..

 

[WRFM426]

Side bar question: I’m thinking of making a couple of GMRS antennas; what antenna analyzer is recommended?
Is the Nano VNA any good?

 

[prcguy]

There are many versions but they all seem to provide good information and the accuracy is above most hobby grade test equipment.

Side bar question: I’m thinking of making a couple of GMRS antennas; what antenna analyzer is recommended?
Is the Nano VNA any good?

 

[Project25_MASTR]

I bounce between Digikey and Mouser for small parts but not antennas.

If Mouser has it in stock...I typically purchase it from there. Of course, I'm local so I pickup locally and avoid shipping.

Antennas I tend to order from Tessco but that is also partially due to convenience of having the option to pickup locally as well.

 

[WRFM426]

Another question… mounting the FG4607 - is it OK to use 1” pole, or is 1 ¼” better?
Antenna weighs 7kg, I believe..

 

[prcguy]

I would go 1 1/4" minimum and only if there is maybe 5ft of unguyed mast. If there will be 10ft of unguyed mast I would go at least 1 1/2". The mounting pipe on the antenna is close to 1 1/4" dia.

Another question… mounting the FG4607 - is it OK to use 1” pole, or is 1 ¼” better?
Antenna weighs 7kg, I believe..

 

[ramal121]

Be aware the FJ???? antennas do not come with a clampset or any kind of mounting. Make sure whatever clampset you choose will fit the antenna and mast.

 

[WRFM426]

Be aware the FJ???? antennas do not come with a clampset or any kind of mounting. Make sure whatever clampset you choose will fit the antenna and mast.

Good point; I did however order an appropriate mounting kit.
Just now ‘patiently’ awaiting delivery…

 

[prcguy]

Two stainless steel hose clamps are more than adequate for the 4607. The fiberglass will break before the clamps give out.

Be aware the FJ???? antennas do not come with a clampset or any kind of mounting. Make sure whatever clampset you choose will fit the antenna and mast.

 

[jeepsandradios]

The 4607 with standard mounting kit will fit a 1 /14" galvanized pipe just fine. I used to buy it by the 10' piece as the ASP would screw on it also. We went thru a ton of it at the shop.

 

[Project25_MASTR]

The 4607 with standard mounting kit will fit a 1 /14" galvanized pipe just fine. I used to buy it by the 10' piece as the ASP would screw on it also. We went thru a ton of it at the shop.

1.25" EMT is what I typically used. Cheap and works well. For a wall mount I'd typically pair it with an WM-4 mount or the larger one if I needed to get around an eve. Can't tell you how many control station setups I built like that...